The replacement of chlorine in organochlorine compounds with hydrogen by means of electrochemical reduction is a known and valuable process. 2,3,5,6-Tetrachloropyridine and 2,3,5-trichloropyridine, which are important intermediates for the production of insecticides, herbicides and the like, for example, are known to be prepared by the electrochemical reduction of pentachloropyridine (U.S. Pat. No. 3,694,332) and 2,3,5,6-tetrachloropyridine (U.S. Pat. No. 4,242,183), respectively. In a similar manner, 3,6-dichloropicolinic acid is known to be prepared from tetrachloropicolinic acid or 3,5,6-trichloropicolinic acid (U.S. Pat. No. 4,217,185).
The development of commercial processes based on electrochemistry is highly dependent upon the development of electrochemical cells that are efficient with respect to electrical energy utilization, can be constructed for a reasonable price, have a long service life and which selectively facilitate the desired reaction. Cells that are useful for the replacement of chlorine in organochlorine compounds with hydrogen involve at the minimum: a cathode at which the electrochemical dechlorination takes place, an anode at which water is converted to oxygen, and an electrolyte which initially contains the organochlorine compound to be reduced.
The electrochemical cells which have been reported to-date for use in processes in which chlorine in organochlorine compounds is replaced with hydrogen have proven to be unsatisfactory with respect to the anodes employed. The graphite anodes disclosed in U.S. Pat. No. 4,217,185 were found to be very sensitive to the type of graphite involved and suffered from a tendency to spall and to lose activity and selectivity in use. They further tend to contain traces of heavy metal impurities which leach into the electrolyte and inactivate the cathode. Electrochemical cells using graphite anodes were, accordingly, found to have a short service life. The stainless steel anodes disclosed in U.S. Pat. No. 4,533,454 were found to corrode at an unacceptably high rate. This corrosion not only damages the anode, but also releases heavy metal ions into the electrolyte which inactivate the cathode. As a consequence, cells containing stainless steel anodes also have relatively short service lives.
The discovery of new anodes for the electrochemical replacement of chlorine in organochlorine compounds by hydrogen cells is, therefore, of great interest. Suitable anodes should be (1) resistant to spalling and dimensionally stable, (2) resistant to corrosion (a) in aqueous alkaline media containing chloride ion; (b) in concentrated hydrochloric acid, and (c) when cycled between cathodic and anodic potentials, (3) inert with respect to contaminating the electrolyte and cathode with heavy metal ions, (4) active in producing oxygen from aqueous solutions containing chloride ion, and (5) able to cooperates with a suitable cathode to selectively replace chlorine in organochlorine compounds with hydrogen.